Conventional solid oxide fuel cell (SOFC) generators typically include tubular fuel cells arranged in a grouping of rectangular arrays. Each fuel cell has an upper open end and a lower closed end, with its open end extending into a combustion zone. A typical tubular fuel cell has a cylindrical inner air electrode, a layer of electrolyte material covering most of the outer surface of the inner air electrode, and a cylindrical fuel electrode covering most of the outer surface of the electrolyte material. An interconnect material extending along the length of the fuel cell covers the circumferential segment of the outer surface of the air electrode which is not covered by the electrolyte material. An electrically conductive strip covers the outer surface of the interconnect material, and allows electrical connections to be made to an adjacent fuel cell or bus bar. The air electrode may comprise a porous lanthanum-containing material such as lanthanum manganite, while the fuel electrode may comprise a porous nickel-zirconia cermet. The electrolyte, which is positioned between the air and fuel electrodes, typically comprises yttria stabilized zirconia. The interconnect material may comprise lanthanum chromite, while the conductive strip may comprise nickel-zirconia cermet. Examples of SOFCs are disclosed in U.S. Pat. No. 4,431,715 to Isenberg, U.S. Pat. No. 4,490,444 to Isenberg, U.S. Pat. No. 4,562,124 to Ruka, U.S. Pat. No. 4,631,238 to Ruka, U.S. Pat. No. 4,748,091 to Isenberg, U.S. Pat. No. 4,791,035 to Reichner, U.S. Pat. No. 4,833,045 to Pollack et al., U.S. Pat. No. 4,874,678 to Reichner, U.S. Pat. No. 4,876,163 to Reichner, U.S. Pat. No. 5,108,850 to Carlson et al., U.S. Pat. No. 5,258,240 to Di Croce et al., and U.S. Pat. No. 5,273,838 to Draper et al., each of which is incorporated herein by reference.
During operation of the fuel cell generator, air is provided to an inside air electrode of each tubular cell, and hydrogen-rich fuel is supplied to an outside fuel electrode surface. The fuel and oxidant are utilized electrochemically to produce electrical energy. The depleted air, comprising about 16 percent oxygen, exits the open end of the cell, and the spent fuel of low hydrogen concentration is eventually discharged into a combustion area surrounding the cell open ends.
During normal run conditions, the fuel gas entering the SOFC combustion zone has a low concentration of hydrogen due to the fuel being consumed within the cell stack. In addition, a relatively large amount of oxygen depleted air exits the cells, keeping the air/fuel ratio well beyond stoichiometric in the combustion plenum. This helps to keep the combustion zone temperature at approximately 950.degree. C., well within the allowable range for the cells. In addition, the high volumetric flow of air out of each cell may be sufficient to protect the air electrode and open end from any risk of hydrogen reduction.
However, during certain generator stop conditions with the stack in an open circuit condition, e.g., loss of grid connection, the air supply may be reduced to a maximum of about 10 percent or less of the normal airflow. The fuel flow to the generator is replaced with a reducing purge flow which serves to protect the fuel electrode from oxidation. This purge flow also causes any stored fuel within the generator to be pushed into the combustion zone where it burns with the available air. There are two primary concerns with this situation. First, the air/fuel ratio is closer to stoichiometric and will result in more combustion and a hotter combustion zone temperature. Second, the reduced air flow leaving each cell may not be sufficient to completely protect the open ends of the cells from hydrogen reduction. Either of these problems have the potential for causing damage to the fuel cells.
The above-noted problems may be reduced or eliminated if the hydrogen can be prevented from reaching the combustion zone in any significant quantity. The present invention has been developed in view of the foregoing and to address other deficiencies of the prior art.